Circuit arrangement for controlling the beam of a cathode-ray tube



8, 1970 G. w. BRQEKEMA 3,525,011

CIRCUIT ARRANGEMENT FOR CONTROLLING THE BEAM OF A CATHODERAY TUBE Filed Oct. 24, 1968 2 Sheets-Sheet 1 um iiiiii""""" ""Wlilllllll'""lllllil b FIG!- INVENTOR. GERHARD WBROEKEMA 711M ALJT4 AGENT 8, 1970 G. w. BROEKEMA 3525,01]

. CIRCUIT ARRANGEMENT FOR CONTROLLING THE BEAM OF A CATflODE-RAY TUBE Filed 001;. 24, 1968 2 Sheets-Sheet 2 v INVENTOR. GERHARD w .BROEKEMA BY 20M mlfi United States Patent US. Cl. 315-30 11 Claims ABSTRACT OF THE DISCLOSURE A CRT control circuit comprising a first capacitor coupled between the output of a source of control voltage and the control grid of the CRT. A chopper has its input coupled to the output of the control voltage source and supplies to the control grid, via a second capacitor and a first resistor in series, amplitude modulated voltage pulses. A clamping diode shunted by a second resistor is connected at one end to the cathode circuit of the CRT and at the other end to the junction between said second capacitor and first resistor. The second capacitor and second resistor are chosen to have an RC time constant that is large relative to the recurrence period of the chopper voltage pulses.

This invention relates to a circuit arrangement for controlling, that is to say for blanking or releasing, the beam of a cathode-ray tube by controlling the voltage at the control grid of this tube, the high-frequency components of the control voltage being applied to said control grid through a first capacitor and its low-frequency and direct voltage components through a grid resistor.

Such arrangements are known, for example, from the book Oscilloscope Measuring Technique by J. Czech, 1965, more particularly Figs. 4-65 and the associated text on pages 126 and 127. They have considerable ad vantage when compared with other known arrangements in which the control voltage is applied to a special blanking electrode of the cathode-ray tube. A cathode-ray tube without a special blanking electrode is considerably cheaper and generally shorter than a corresponding tube having a blanking electrode. When the beam is blanked, the background illumination is negligibly small.

The peak beam current and hence also the rate of writing can be increased to a higher value.

The control does not give rise to displacement of the luminous spot and the efliciency of the electron gun is higher.

They have the disadvantage, however, thatthe transmission of the low-frequency components, and particularly of the DC. component, of the control voltage to the first grid of the cathode-ray tube presents a problem because this grid, like the cathode, is always accurately at a high negative potential with respect to ground so that this transmission can only be efiected by means of a floating high voltage source.

One object of the invention is to provide an advantageous and inexpensive solution of this problem.

The arrangement according to the invention is characterized in that it comprises a chopper to which the control voltage is applied and which produces voltage pulses amplitude-modulated by the control voltage. A second capacitor is connected between the output of the chopper and a clamping diode shunted by a discharge resistor and through which said second capacitor is connected to a point in the cathode circuit of the tube. The second capacitor with its discharge resistor constitute an RC-netice Work having a large time constant with respect to the recurrence period of the chopper voltage pulses. A grid resistor is connected to the junction of the second capacitor and the clamping diode. In operation, the clamping diode passes only the tops of the chopper voltage pulses so that the second capacitor remains charged with a voltage substantially equal to that at the said point in the cathode circuit of the tube.

Especially a capacitor having a considerable capacitance and a high insulation voltage can thus be economized, while the first capacitor together with the grid resistor constitute a low-pass filter, by which the chopper voltage pulses can be eliminated and by which moreover the ripple voltage components of the high voltage cathode supply, operative between the cathode and the first grid of the cathode-ray tube, can be considerably attenuated, so that the smoothing filter of this voltage source can also be economized. Furthermore, the radiation intensity can be readily controlled at a low voltage level in different ways.

The invention will be described more fully with reference to the drawing, in which:

FIG. 1 is the basic circuit diagram of a known circuit arrangement of the kind to which the invention relates,

FIG. 2 shows the circuit diagram of a known embodiment of the separate floating high voltage source of the circuit arrangement shown in FIG. 1,

FIG. 3 shows the basic circuit diagram of a circuit arrangement according to the invention,

FIG. 4 shows a voltage-time diagram for explaining the operation of the circuit arrangement of FIG. 3,

FIG. 5 shows the circuit diagram of a first embodiment of this circuit arrangement, and

FIG. 6 shows the circuit diagram of a second embodiment of a circuit arrangement in accordance with the invention.

The known circuit arrangement shown in FIG. 1 serves to blank and to release the beam of a cathode-ray tube 1, the cathode of which, in operation, is kept at a high negative potential of, for example, 2000 v. with respect to the grounded anodes of the electron gun by means of a voltage source 2. The arrangement comprises a control amplifier SV having an output stage which is supplied with a positive voltage via a load resistor 3. The electron beam is blanked or released under control of the control voltage at the output terminal A of this amplifier, the other terminal of which is grounded. The high-frequency components of the control voltage are transmitted to the first grid of the cathode-ray tube through a first capacitor 4, whereas the low-frequency components and the directvoltage component of this voltage are applied to said grid through a D.C.-circuit comprising the series-combination of a resistor 5, a voltage source 6 having a voltage sub stantially equal to that of the high-voltage source 2, and a grid resistor 7.

The voltage source 6 serves to transmit the control signal produced at the point A to the first grid (point B), at a direct-voltage level approximately equal to that of the cathode of the tube 1. This involves a variation of the potential with respect to ground of this whole voltage source, with the control voltage at point A. This voltage source is thus a floating voltage source. The use of such a floating voltage source generally has disadvantages. In the present case, the main disadvantage is the capacitance with respect to ground of the voltage source 6 (shown in dotted lines as a capacitor 8).

Due to the presence of the stray capacitance 8, the resistors 5 and 7 must have high values in order to prevent a capacitative load on the control amplifier SV. Consequently, the resistor 5 constitutes, together with the capacitance S, a filter passing the direct voltage and the voltages of low frequencies, whereas the capacitor 4 constitutes, together with the resistor 7, a filter passing the high frequencies between the points A and B.

In practice, the voltage source 6 comprises, as shown in FIG. 2, a high-voltage rectifier tube 9, the cathode of which is connected to an additional winding 10 of a supply transformer for the tube 1, for example, of the highvoltage supply transformer for the cathode of said tube. The other end of the winding 10 constitutes the positive terminal of the voltage source 6, the negative terminal of which is constituted by the anode of the tube 9. A smoothing capacitor 11 is connected between these two terminals.

The cathode of the tube 9 and the end of the winding 10 connected thereto have a non-negligible stray capacitance with respect to ground, shown in dotted lines as a capacitor 12, and this results in an alternating-voltage component being superimposed on the signal to be transmitted to point B. In order to prevent the electron beam of the tube 1 from being modulated in a disturbing manner during observation by this alternating-voltage component, the capacitance 8 of the whole voltage source 6 with respect to ground must be chosen to be considerably larger than the stray capacitance 12. The ultimate consequence is that the capacitor 4 must have an objectionably high value so that it occupies a large space and is very expensive, because it must be insulated for the voltage of the source 2.

In a practical example, the stray capacitance 12 was of the order of 10 pf.

The resistor 5, which must be considerably larger than the internal resistance of the output stage and hence larger than its load resistance 3 of, for example, 3.3 KS2, had a value of IOOKQ, so that a ripple voltage of 10 v. at the positive terminal of the source 6 corresponded to an admissible ripple voltage of 0.3 v. at the point A.

In order to keep the ripple voltage across capacitor 8 below 10 v., this capacitor had to have a value of 2000 pf.

Finally, the resistor 7 must be small with respect to the input resistance R between the first grid and the cathode of the tube 1 and it was chosen to be equal to 1M9. On the other hand, since the time constant of the RC-network 4, 7 must be considerably, for example, 200 times, larger than that of the network 5, '8 the value of the capacitor 4 was chosen to be 0.047 t.

The arrangement according to the invention, shown diagrammatically in FIG. 3, comprises a converter or chopper CH to which the control voltage from the control amplifier SV is applied and which produces voltage pulses the amplitude of which is modulated by this control voltage. If, for example, the control signal is a square-wave voltage having comparatively short positive pulses, as represented by the lower full lines of the diagram of FIG. 4a, a signal such as represented by the whole diagram of FIG. 4a is produced at the output of the chopper CH, i.e. at point C. The direct voltage level indicated by the dotand-dash line n corresponds to an extreme state of the chopper, for example, the state of maximum conductivity, and lies in the proximity of a constant potential, for example, ground potential. These chopper voltage pulses, amplitude-modulated by the control voltage, are transmitted to the first grid of the tube 1 (point B) through a capacitor 13 and the grid resistor 7. A clamping diode 14, shunted by a discharge resistor 15, is connected between the cathode of the tube 1 and the junction D of the capacitor 13 and of the grid resistor 7. The diode 14 is connected so that it is capable of passing the chopper voltage pulses, which are positive in the present case. The discharge resistor 15 is smaller than the leakage resistance between the first grid and the cathode of the tube 1. Consequently, the level of point D corresponding to the line 21 of the diagram of FIG. 4a is stabilized on the direct voltage level of the cathode of the tube 1, as indicated by the dot-and-dash link k of the diagram of FIG. 412. After each chopper voltage pulse, the diode 14 is cut off and the point D again becomes strongly negative with respect to the level k, whereupon its potential increases again comparatively slowly by the discharge of capacitor 13 through the discharge resistor 15. The signal transmitted to point D has the form shown by the diagram of FIG. 4b, the capacitor 13 acting as a direct voltage source between successive chopper voltage pulses. The resistor 15, through which the capacitor 13 can be discharged, prevents the diode 14 from being cut off by the small leakage current flowing between the cathode and the first grid of the tube 1.

Provided that the output impedance of the control amplifier SV, and hence the resistor 3, has a small value, the chopper voltage pulses and the ripple produced by the discharge of the second capacitor 13 through the discharge resistor 15, appearing between said cathode and said first grid, are substantially filtered out by the RC- network comprising the grid resistor 7 and the first capacitor 4, through which the first grid of the tube 1 is capacitively grounded via the low output impedance of the control amplifier SV, so that the orginial control voltage indicated by the full line of the diagram of FIG. 4a appears again at point B (first grid of the tube 1). On the other hand, and leaving the chopper voltage pulses out of consideration, the control voltage at point D has the same amplitude as at point A, so that the RC- network 4, 7 cannot constitute a low-pass filter for this control voltage because the voltage across the capacitor 4 does not vary appreciably.

The direct voltage level of the control voltage at point B with respect to the cathode of the tube 1 is determined by the amplitude of the chopper voltage pulses and can be readily changed at a low-voltage level, for example, by adjustment of the chopper itself and/or of the voltage applied to the point A through resistor 3. The required control range is then determined by the cut-off value of the voltage between the first grid and the cathode of the cathode-ray tube used, and also by the releasing and blanking values of the control voltage operative at point A. The amplitude of the chopper voltage pulses should be at least a few volts larger than the difference between the blanking and the releasing values of the control voltage.

The time constant of the RC-network comprising the first capacitor 4 and the grid resistor 7 should be large with respect to the duration of each of the voltage pulses produced by the chopper and this condition can be filled the more readily and with smaller values of the elements of the RC-network as the voltage pulses produced by the chopper are shorter with respect to their period of recurrence. An oscillation having a short sweep and a long return period can be obtained by a particular construction and/or proportioning of the chopper.

On the other hand, the ripple component of the high voltage applied to the cathode of the tube 1 is also applied to the first grid of this tube through the diode 14 and the discharge resistor 15, and through the grid resistor 7, so that the ripple voltage operative between the first grid and the cathode of said tube decreases with the value of the first capacitor 4, through which the first grid is decoupled with respect to point A and, through the control amplifier SV, with respect to ground.

The capacitance of the first capacitor 4 is preferably of the same order of magnitude, or even of the same value, as that of the second capacitor 13.

In the first embodiment shown in FIG. 5, the highvoltage source 2 is shunted by a decoupling capacitor 16 and by a potentiometer 17, 18, 19, by means of which an adjustable positive bias voltage is applied to the third grid of the tube 1. The chopper includes a pnp-type transistor 20, the emitter of which is connected to a source E of adjustable bias voltage through the resistor 3 and to point A, i.e. to the output terminal of the control ampli fier SV, through a resistor 21. The collector of this transistor is connected to the negative terminal of a supply voltage source of, for example, 100 v. through resistors 22 and 24 and its base is grounded and connected to its emitter through a diode 24 connected in the reverse direction as a protection against reverse pulses which may occur when the arrangement is switched on or off. Between the emitter and the base of transistor 20 are applied short positive control pulses derived from a direct voltage converter of the oscillograph or other apparatus equipped with the cathode-ray tube 1. These pulses are derived through a differentiating RC-network comprising a series capacitor 25, a parallel resistor 26, a diode 27 passing only the positive peaks, and a limiting resistor 28.

The transistor 20, the base of which is grounded, constitutes a current source with respect to point C of the arrangement. Since it is continuously conducting, its emitter input impedance is very low, so that the amplitude of the chopper voltage pulses at this point is also low, for example, of the order of 0.5 v. On the other hand, the output impedance R of the control amplifier SV is also very low, so that the chopper'voltage pulses of small amplitude operative at the emitter of transistor are attenuated at point A by a factor RA+R21 R representing the value of the emitter resistor 21 of the transistor 20.

The recurrence frequency of the control pulses derived from the direct voltage converter is comparatively low, for example, equal to 20 kc./s. Consequently, the capacitance of the capacitors 4 and 13 is not very small, for example, equal to 4700 pf. With such a capacitance, the direct voltage produced by the chopper voltage pulses across the capacitor 13 is very high, even when the highvoltage source 2 is switched off, so that the diode 14 could be destroyed by switching off the source 2. A glow discharge tube 29 acting as a direct voltage limiter is therefore connected across the parallel-combination of the diode 14 and the discharge resistor 15, so that the reverse voltage across the diode is limited to the ignition voltage of the glow tube, which is lower than the maximum permissible reverse voltage for the diode 14.

The duration of the voltage pulses produced by the chopper is determined by the collector resistors 22, 23 and the collector capactance of the transistor 20, plus the capacitance of the diode 14 and the stray capacitances (wiring etc.).

The time constant of the RC-network 7, 4 is determined by the smoothing required for filtering out the chopper voltage pulses. If, for example, the maximum input impednace of the tube 1 between the first grid and the cathode is equal to 25MS2, the value of the grid resistor 7 should be at the most equal to 0.5MS2. With a capacitor 4 of 5000 pt. and chopper voltage pulses having a duration of 2 microseconds and an amplitude of 60 v., the amplitude of the remaining ripple voltage across the capacitor 4 becomes equal to 48 mv., which value is still permissible.

In a practical embodiment of the arrangement shown in FIG. 5, the control amplifier SV supplied a voltage varying between +40 and +10 v. and the direct voltage converter 0 supplied voltage pulses having a peak-to peak amplitude of 20 v. The voltage of the high-voltage source 2 was 2000 v., and that of the supply voltage source for the transistor 20 was 100 v. The bias voltage at point E could be adjusted between and +30 v.

The transistor 20 was of the Philips type AP 118, the di- 6 of 4700 pf. and the capacitor 25 had a value of 560 pf. The resistors had the following values:

Due to the comparatively low recurrence frequency of the chopper voltage pulses and to the resulting comparatively high capacitance of the capacitors 4 and 13, the major part of the ripple voltage of the high-voltage source 2 was also operative between the cathode and the first grid of tube 1, so that the source 2 had to be provided with a good smoothing filter.

By adjustment of the collector resistance of the transistor 20 by means of the resistor 23, various amplitude losses of the control voltage variations can be compensated for, so that the votlage variations transmitted through the chopper, the capacitor 13 and the resistor 7 to the first grid of the tube 1 ultimately have the same amplitudes as those directly transmitted by the capacitor 4 from point A to point B.

A first loss of amplitude is due to the fact that the emitter-collector current amplification factor of the transistor 20 is lower than 1.

A second loss is due to the finite discharge time constant of the capacitor 13 through the resistor 15. The difference of charge of the capacitor 13 between two successive chopper voltage pulses represented by the oblique lines in the diagram of FIG. 4b increases with the amplitude of the chopper voltage pulses. These small sawtooth pulses are superimposed on the control voltage, as well as the chopper voltage pulses, and are filtered out by the low pass filter 7, 4, but due to integration, the amplitude of a control voltage step is reduced approximately by the difierence between the amplitudes of the respective sawteeth corresponding to the relevant values.

A third loss is due to filtering of the chopper voltage pulses because the amplitude of these peaks, which have to be integrated, varies with the control voltage itself.

Finally, the control voltage at point B is attenuated in the proportion where R represents the value of the grid resistor 7 and R the value of the input impedance between the first grid and cathode of the tube 1.

The control voltage variations at point D must therefore have a larger amplitude than the variations desired at point B, and the variations at point C must in turn have a still larger amplitude, which is achieved by means of the adjustable resistor 23 by which the collector resistance 22, 23 of the transistor 20 can be made to exceed the value of its emitter resistor 21. This adjustment was effected in the presence of positive (releasing) control voltage pulses having a recurrence period of 20 milliseconds.

The brightness of the cathode spot of the tube 1 can be adjusted in different ways as by variation of the collector supply voltage of the transistor 20, by adjustment of the direct voltage level of the control voltage in the control amplifier SV and, as shown, by variation of the direct voltage level of the emitter of the transistor 20 by means of the bias voltage source connected to point E. Irrespective of the manner in which the adjustment takes place, it is to be ensured that, with any value of the control voltage occurring in operation, chopper voltage pulses are superimposed thereon and appear at point C; the control voltage alone should not be able to completely cut off nor to completely release the transistor 20. It was found that the mode of adjustment that had the smallest influence on the amplification adjustment was by means of the resistor 23.

The second embodiment shown in FIG. 6 mainly differs from that of FIG. in that the chopper is self-generating. Furthermore, for illustration of possible modifications, the pup-type transistor was replaced by an npn-type transistor 20. This involves a reversal of the chopper output signals (FIG. 4) and of the diode 14, so that a negative bias voltage is required for the first grid of the tube 1 with respect to its cathode. This bias voltage is produced across a Zener diode connected between the negative terminal of the high-voltage source 2 and the cathode of the tube 1, which diode is shunted by the parallel-combination of an electrolytic capacitor 31 and a resistor 32. This resistor is also utilized for a brightness control at high-voltage level, point D being connected through the diode 14 and the resistor 15 to an adjustable tapping on the former resistor, which tapping is in turn decoupled by a capacitor 33.

Due to the fact that the brightness control is effected in a different manner at high-voltage level, an adjustment of the direct voltage level of the emitter of the transistor 20 now is superfluous.

The collector of the transistor 20' is connected through the resistors 22 and 23 to the positive terminal of a supply voltage source of, for example, 100 v. Its base is directly connected to the collector of a second transistor 34 of the opposite pup-type with which the transistor 20' is included in a kind of astable multivibrator. The latter includes a a capacitor 35 connected between the respective emitters of the transistors 20' and 34, which capacitor is slowly charged through a resistor 36 connected to the positive terminal of the supply voltage source and is abruptly discharged again through the transistor 34. For this purpose, the base of this transistor is connected to ground, as is its collector through a load resistor 37. The collector of the transistor 20' is connected via a second clamping diode 41 to the tapping of a potentiometer connected between the positive terminal of the voltage supply source and ground, and comprising two resistors 38 and 39. The second clamping diode 41 limits the amplitude of the chopper voltage peaks and fixes their negative peak level at point c, said tapping being decoupled by a capacitor 40.

The astable multivibrator described is particularly suitable to produce very short negative chopper voltage pulses separated by comparatively long intervals. Such a narrow form of the chopper voltage pulses is very desirable because the loss of amplitude of the control voltage variations between the points C and D due to the integration is thus considerably reduced. The transistor 20 is always more or less conducting because the direct voltage level of the tapping of the potentiometer 38, 39 and of the collector of the transistor 34, for example, in the cut off condition, exceeds the maximum voltage level of its emitter, which varies with the voltage level at point A. As regards the control voltage applied to its emitter through the resistor 21, this transistor therefore operates between the chopper voltage pulses as an amplifier in grounded base arrangement.

At the instant at which the emitter of transistor 34 becomes positive with respect to its collector due to the discharge of capacitor 35 through resistor 36, this transistor becomes conducting. The capacitor 35 discharges through the emitter-collector electrode path of transistor 34 and through the load resistor 37, and also through the base-emitter path of the transistor 20. This transistor thus becomes much more strongly conducting so that its emitter also becomes slightly more positive. The increase in potential of this emitter is transmitted through the capacitor 35 to the emitter of transistor 34, which in turn becomes more strongly conducting, and so on, until the loop amplification across transistors 20 and 34 becomes smaller than 1 due to the discharge of capacitor 35. The emitter of transistor 34 again becomes negative with respect to its base so that the pnp-transistor 34 is cut off. This cutting-off also occurs very abruptly.

On the other hand, the voltage across the collector resistors 22 and 23 cannot exceed that across the resistor 38 of the potentiometer 38, 39 plus the threshold voltage of the clamping diode 41, which consequently determines and stabilizes the collector potential of the transistor 20' during its short periods of high conductivity and prevents this transistor from being saturated, which would result in the recurrence frequency of the produced chopper voltage pulses becoming dependent upon the control voltage.

In a practical embodiment of the arrangement shown in FIG. 6, the recurrence frequency of the chopper voltage pulses produced by the astable multivibrator comprising the transistors 20 and 34 was of the order of 200 kc./s. As a result, the value of each of the capacitors 4 and 13 could be reduced to pf. so that practically the Whole ripple component of the high voltage of the source 2 supplied to the cathode of the tube 1 was then also transmitted to point D and, through the grid resistor 7, was also operative at point B and at the first grid of said tube. Thus, the ripple voltage operative between the first grid and the cathode of the tube 1, and originating from the source 2, was reduced by a factor 20, which permitted a considerable saving in the high voltage filter. It was further found that a limiting element, such as the glow discharge tube 29 of FIG. 5, could be dispensed with because the amount of energy required for charging the capacitors 4 and 13 was too small to produce across the diode 14 and the resistor 15 a reverse voltage peak that could damage the diode when the chopper was switched off.

The Zener diode 30 was of the Philips type BZY 94/C 75, the transistor 20' was of the type BSX 21, the transistor 34 of the type BC 187 and the second clamping diode 41 of the type BAX 16. The capacitors and resistors had the following values:

Capacitor 3322,000 pf. Capacitor 35-3300 pf. Capacitor 4016,uf. Resistor 36l20K!2 Resistor 37-470S2 Resistor 38-12KQ Resistor 391KQ As a matter of course, many modifications of the two embodiments described are possible. For example, in the embodiment of FIG. 5, the transistor 20 may be replaced by an npn-transistor 20', and in the embodiment of FIG. 6, the transistor 20 may be replaced by a pnptransistor 20. In each of these embodiments, the brightness control and/ or the bias of the first grid of the tube 1 may be efiected by any of the methods described or even briefly mentioned.

What is claimed is:

1. In a circuit arrangement for controlling the beam of a cathode-ray tube by means of a control voltage applied to the control grid of said tube, in which the highfrequency components of the control voltage are applied to said control grid through a first capacitor and its lowfrequency and direct voltage components through a grid resistor, the improvement comprising a chopper to which the control voltage is applied and which produces voltage pulses amplitude-modulated by the control voltage, a second capacitor, a clamping diode shunted by a discharge resistor and connected at one end to a point in the cathode circuit of the tube, means connecting said second capacitor between the output of the chopper and the other end of said diode, the second capacitor with its discharge resistor constituting an RC-network having a large time constant with respect to the recurrence period of the chopper voltage pulses, and means connecting the grid resistor to the junction of the second capacitor and the clamping diode whereby, in operation, the clamping diode passes only the tops of the chopper voltage pulses so that the second capacitor continues to be charged with a voltage substantially equal to the voltage at the said point in the cathode circuit of the tube.

2. A circuit arrangement as claimed in claim 1 Wherein the cathode-ray tube is supplied with a high voltage produced by means of a voltage converter, and means for driving the chopper by pulses derived from said converter.

3. A circuit arrangement as claimed in claim 1 wherein the chopper includes a transistor, a resistor, means including said resistor for applying to the emitter of the transistor a forward control voltage, means for applying control pulses between the emitter and the base of the transistor, and means connecting the collector of said transistor to a supply voltage source through a load resistor and to the clamping diode through the second capacitor.

4. A circuit arrangement as claimed in claim 3 further comprising means for making the chopper self-generating that includes a second transistor of a conductivity type opposite to that of the first transistor, means connecting the collector of said second transistor to the base of the first transistor and also to a point of constant potential through a load resistor, means connecting the emitter of the second transistor to the emitter of the first transistor through a third capacitor and also to said supply voltage source through a clamping resistor, means connecting the base of the second transistor to a point of constant potential, and means connecting the collector of the first transistor to a point of lower reverse potential through a second clamping diode.

5. A circuit arrangement as claimed in claim 3 wherein the chopper includes at least one D.C. bias circuit for adjusting the rest potential of the emitter and/or the base of the first transistor.

6. A circuit arrangement as claimed in claim 1 wherein the capacitance of the second capacitor is chosen to be of the same order of magnitude as that of the first capacitor.

7. A circuit arrangement as claimed in claim 1 wherein the time constant of the RC-network comprising the' first capacitor and the grid resistor is large with respect to the duration of each of the voltage pulses produced by the chopper.

8. A circuit arrangement as claimed in claim 7 wherein the duration of each of the voltage pulses produced by the chopper is short with respect to the recurrence period of said voltage pulses.

9. A control circuit for the control grid of a cathode ray tube comprising, a source of control voltage, a first capacitor directly coupling the output of said control voltage source to said control grid, a chopper having its input coupled to the output of said control voltage source, said chopper including an output circuit at which voltage pulses are produced that are amplitude modulated by said control voltage, a second capacitor, a first resistor, means serially connecting said second capacitor and said first resistor between the output of the chopper and said control grid, a diode and a second resistor connected in parallel, and means connecting the parallel combination of the diode and the second resistor between the junction of said second capacitor and said first resistor and the cathode circuit of the cathode ray tube.

10. A control circuit as claimed in claim 9 wherein the cathode side of the diode is connected to the cathode circuit of the cathode ray tube, and wherein the second capacitor and the second resistor are chosen to have a large time constant relative to the recurrence period of said voltage pulses.

11. A control circuit as claimed in claim 10 wherein the RC network comprising the first capacitor and the first resistor are chosen so as to produce a time constant that is large relative to the duration of each of said voltage pulses.

No references cited.

RODNEY D. BENNETT, JR., Primary Examiner- J. G. BAXTER, Assistant Examiner 

